Insulated metal sheath heating element for electric water heaters



June 6, 1967 F. E. CHENEY ET AL 3,324,280

INSULATED METAL SHEATH HEATING ELEMENT FOR ELECTRIC WATER HEATERS FiledAug. 6, 1964 I NVENTORS ATTORNEYS United States Patent 3,324,280INSULATED METAL SHEATH HEATING ELE- MENT FOR ELECTRIC WATER HEATERSFrank E. Cheney, 109 St. Martins Road, Cherry Hill, NJ.

08034, and Frank Kahn, 1865 Edmund Road, Abington, Pa. 19001 Filed Aug.6, 1964, Ser. No. 387,923 6 Claims. (Cl. 219-544) ABSTRACT OF THEDISCLOSURE This invention comprises a rod-type copper-sheathed immersionheating element for an electric water heater, having a plating of nickelon the external surfaces of the copper. The nickel surface is sanded toprovide a matte surface and heated to deposit an oxide coating. Anexternal insulating coating of polytetrafluoroethylene is then applied.The purpose of this new heating element is to minimize the currentflowing to the heating element from the galvanic anode used for cathodicprotection of a glass-lined Water heater tank.

The present invention relates to heating elements for electric waterheaters and more particularly to rod-type metal sheath immersionelectric heating elements which are resin-coated to insulate the metalsheathing from contact with Water. The invention also relates to methodsof electrically insulating the metal sheath of an electric immersionwater heater element.

A purpose of the invention is to apply a resin-coating to the metallicsheath of an electric immersion Water heater element which will becapable of withstanding the severe heat shock in cyclic operation andremain serviceable for a long period of time.

A further purpose is to coat a nickel external layer of the metallicsheath by polytetraflu-oroethylene which will remain adhering thereto.

A further purpose is to nickel-plate the copper sheath of an electricwater heater element and then apply polytetrafluoroethylene to thenickel-plating.

Further purposes appear in the specification and in the claims.

In the drawings we have chosen to illustrate one only of the numerousembodiments in which the invention may appear, selecting the form shownfrom the stand points of convenience in illustration, satisfactoryoperation and clear demonstration of the principles involved.

FIGURE 1 is a conventional central vertical section of 3,324,280Patented June 6, 1967 sheath'which needs no cathodic protection becausethe corrosion of copper is ordinarily negligible in domestic waterheaters. In the case of the tin plating, the current may be reduced byabout but is still considered excessive.

The life of the sacrificial anode could be greatly prolonged and theuseful life of the water heater could be proportionately extended if theprotective current which is unnecessarily diverted to the bare coppersheath of the immersion heating element could be minimized. UnitedStates Patent No. 2,649,532 issued to Kenneth L. Woodman on Aug. 15,1953, for Water Heater Apparatus, proposes to accomplish this by coatingthe entire surface of the sheath of the immersion electric heating unitwith an insulating coating which is resistant to hot waer. This patentstates that the insulating coating will be a varnish of any well knowntype, such as for example the varnishes then on the market under thetrade names Micarta, Permolite or Heresite.

By contact with Westinghouse Electric Corporation, the assignee of saidWoodman patent, it was learned that on practical tests none of thesecoatings were found to withstand service conditions and commercialexploitation of the invention of this patent was accordingly abandoned.Westinghouse Electric Corporation advised that its B-192 Micartainsulating varnish, a polyvinyl butyral resin product, was deemed to bemost suited for this purpose but proved to be unsatisfactory. In orderto confirm this conclusion, a sample of this Micarta varnish wasobtained, along with a sample of Teresite, a phenol-formaldehyde resinmarketed by Heresite Chemical Company, and a sample of Permolite, asoluble plasticized alkyd resin used as a finish for viscous and acetatefabrics, marketed by Arkansas Company, Inc.

Each of these varnishes was coated on a clean conventional bare coppersheath immersion electric water heating element in accordance with thedirections of the manufacturer of the varnish. Effectiveness of thecoating for the intended purpose was evaluated by making endurance testsof the electric heating elements immersed in tap water and cyclicallyenergized at the rated voltage to provide a thermal cycle of watertemperature between 150 F. and 180 F. with the following results: After100 heating cycles the Micarta varnish had become disbonded in flakesover the entire surface of the heating element. The insulatingresistance which was 300,000

an electric water heater embodying the principles of the invention.

FIGURE 2 is a diagrammatic enlarged section of the electric water heaterelement of FIGURE 1.

Describing in illustration but not in limitation and referring to thedrawings:

In the production of steel tank storage electric water heaters providedwith glass linings, the presence of imperfections in the glass is socommon that it is ordinarily necessary to protect the steel exposed atsuch imperfec tions by a sacrificial anode, usually of magnesium orzinc, which provides cathodic protection. The anode supplies protectivecathodic current to the exposed steel of the tank and is consumed inproviding this protection. Once the anode is completely dissipated,rapid corrosion at exposed metal surfaces occurs. Most rod-typeimmersion heating elements for electric water heaters are sheathed incopper which is left bare or plated with a more electronegative metalsuch as tin. In such Water heaters with bare copper sheaths, up to aboutthree-quarters of the electric current from the anode may flow to thecopper ohms before the test had dropped to 50 ohms after the test.

In the case of the Heresite varnish, at the end of 20 heating cyclesblisters began to form on the surface of the coating, at the end of 30cycles pieces of the coating began to flake off, and at the conclusionof 300 cycles large areas of bare metal were exposed and the remainingcoating was crazed and covered with blisters. The insulation resistancewas 1,800,000 ohms before the test and 400 ohms after the test. In thecase of the Permolite varnish, itwas found to be impossible to applythis material to the bare copper surface to obtain any practicalinsulating value. This is a product normally used in water solution forfinishing various types of textile fabrics.

Similar tests were made without success on a substantial number of otherinsulating coatings. One of those tested was a silicone coating. Inevery case the coating disintegrated or disbonded from the copper underthe testing procedure.

The most promising coating tried was polytetrafluoroethylene coated to athickness of approximately 1 mil. This was applied directly to thecopper surface by dipping the electric immersion heating element in acommercially available polytetrafluoroethylene coating marketed by E. I.du Pont de Nemours & Company under the trade name 3 Teflon 30 AqueousDispersion. The coating was dried and then sintered. This coating alsoblistered and disbonded under tests and was found to be unsatisfactory.

Further efforts were made to produce a satisfactory insulating coatingof polytetrafiuoroethylene on a conventional copper sheath immersionwater heater element using the Teflon 30 coating material above referredto and eventually these efforts were successful.

The coating in question is preferably made according to Ostal U.S.Patent 2,562,118, granted July 24, 1951, for PolytetrafiuoroethyleneCoating Compositions, although optionally it can be made according toU.S. Patent 2,562,117, granted to the same inventor on the same date.

To prepare the surface of the copper to receive thepolytetrafluoroethylene coating it has been found to be important tonickel plate the copper according to any conventional nickel platingprocess. This may be done by cleaning the copper surface by dipping itin a conventional bright dip. This is a brightening pickle composed ofdilute nitric and sulfuric acid, followed by bufling with jewelersrouge, and washing in cold water. The copper is then electroplated withnickel to a thickness of about 0.1 mil to about 1 mil and then washedwith cold water and dried.

The nickel provides a thermal conducting base which has the property offorming a firm adherence with the polytetrafluoroethylene. Conventionalnickel plating is adequate for this purpose and it is not necessary thatthe nickel plating be of uniform thickness.

In the preferred embodiment the nickel surface is slightly roughened asby sand blasting with a fine grit such as aluminum oxide to give asmooth matte surface. The electrode is then heated to a temperature,preferably of the order of 700 to 750 F. to drive off any adheringorganic material which may have become embedded in pores or seams and todeposit an oxide coating which improves the adherence of thepolytetrafluoroethylene. The electrode is then allowed to cool to roomtemperature.

The polytetrafluoroethylene aqueous dispersion used is in the form of aslurry or water dispersion of very small particles ofpolytetrafluoroethylene stabilized with a nonionic wetting agent whichdecomposes cleanly at the fusing temperature of the resin. This materialas supplied by the manufacturer is a hydrophobic negatively chargedcolloid, which comprises about 60% by weight of small particles(averaging of the order of 0.5 micron in diameter) ofpolytetrafluoroethylene suspended in water containing a wetting agent,suitably a non-ionic surface reactant such asoctylphenoxypolyethoxyethanol amounting preferably to about 6% by weightof the polytetratfluoroethylene, although any wetting or dispersingagent may be used provided it is stable in the presence of the otheringredients. The concentration of the wetting agent is not critical. Ifdesired, modifying agents such as pigments, fillers, or the like may beadded provided they are compatible with and stable in the presence ofthe other ingredients and capable of withstanding temperatures in theregion of 750 F.

The water dispersion of the resin is applied to the heating elementsurface by dipping, flowing, sprayed or brushing to a thickness of about1 mil. The water is then removed from the deposited polymer by drying atroom temperature or at moderately elevated temperature, not over 200 F.Finally, the dried polymer is fused or sintered by baking suitable forabout three minutes at a temperature in the range of 675750 F. Sinteringof the resin takes place almost instantaneously on reaching the requiredtemperature and during the remainder of the baking period the stabilizerwetting agent is volatilized and burnt off. The heating element is thencooled slowly to retain good adhesion.

In the drawings we show a conventional electrical water heater 20consisting of a tank 21, which will suitably be internally glass lined(not shown) or otherwise coated for protection on the inside. The tankcontains an inlet connection 22, conveniently located at the bottom andan outlet connection 23, suitably at the top. A sacrificial anode 24 isremovably inserted as by a threaded connec tion at 2 5. The anode 24will conveniently consist of magnesium, zinc or the like,

A looped electrical resistance heater element 26 extends into the tank,and is mounted on a suitable removable base 27 provided with aninsulating support 28 and having electrical connections 30 and 31.

As best seen in FIGURE 2, the electrical resistance heater 26 desirablyconsists of an interior heating element or resistor 32 suitably ofnichrome or other resistance wire, protected by a surrounding insulatinglayer 33, suitably magnesia or other refractory powder, surrounded by acopper sheath 34 as well known in the art. According to the presentinvention a nickel layer 35 surrounds the copper and then apolytetrafluoroethylene layer 36 according to present invention forms abarrier between the copper layer and the water. The interior 37 of thetank will suitably be filled with water, not shown.

Example 1 Three l500-watt copper sheathed heating elements were preparedfor application of the polytetrafluoroethylene coating by firstconventionally nickel plating them to a thickness averagingapproximately 1 mil, lightly sand blasting, heating and allowing them tocool to room temperature as previously described. The elements were theneach coated with two successive coatings comprisingpolytetrafluoroethylene aqueous dispersion, suitably a primer coat and atop coat, each separately applied and fused in the manner previouslydescribed.

The two coating mate-rials used were those commercially available fromE. I. du Pont de Nemours & Co. under the designations 851-1204 TeflonTFE-Fluorocarbon Resin One Coat Green Enamel (primer coat) and 851- 205Teflon TFE-Fluorocarbon Resin Black Enamel (second coat). The Du Pont851-204 product is composed of about 48% of polytetrafluoroethyleneresin soluble by weight in a Water medium and the 851-205 product iscomposed of about 41% polytet-rafluoroethylene solids by weight in awater medium, each containing a nonionic wetting agent as above setforth and a few percent of pigment which is optional. The dispersion wasapplied as a spray to the heating elements, which were supported by theterminal ends, the primer being coated to a thickness of about mil andthe second coat to a thickness of about 1 mil. After fusing, the primercoat had a thickness which ranged from 0.1 mil to about 1 mil andaveraged about 0.35 mil. The second coat after fusing had a thicknesswhich ranged from about 0.1 mil to about 1 mil and averaged about 0.43mil.

The coated electrodes were energized at rated voltage of 236 volts andsubjected to 3,000 heating cycles. Each heating cycle consisted of 15minutes with elements energized and 20 minutes with the elementsde-energized. The water temperature ranged from F. to F. The resistancesof the coating in ohms on the respective electrodw before test were50,000, 60,000 and 70,000 ohms. After test the resistances of thecoating in ohms were 30,000, 40,000 and 30,000 ohms. One of the elementswas then energized at 360 volts (an over-voltage). This gave a wattdensity of 300 watts per square inch as compared to a normal wattdensity of 140 watts per square inch. The element was subjected to 136cycles, consisting of 15 minutes energized and 30 minutes deenergized.Twice, the element was inadvertently energized for a period of 16 hours.The resistance of the coating before test was 20,000 ohms and after testwas 25,000 ohms.

Example 2 The copper sheathing of the heating element is prepared byconventional nickel plating, light sand blasting, heating and allowingto cool to room temperature as previously described. A liquid primercoating corresponding to Example 7 of U.S. Patent 2,562,118 is appliedto a thickness of mil and then dried at a temperature of 200 F. and thenheated at approximately 750 F. for about 5 minutes to produce a fusedcoating. The heating element is then allowed to cool slowly to roomtemperature.

Next a top coat of liquid coating composition E as set forth in saidPatent 2,562,118 made as described in U.S. Patent 2,613,913 is appliedto a thickness of 1 mil. This is dried as above and fused as above andthen allowed to cool slowly to room temperature.

An adhering coating of high insulation value is obtained.

Example 3 The procedure of Example 2 is followed except that the topcoat liquid composition is modified to introduce about 2% of compatibleblack pigment, suitably carbon black producing a black surface. Goodadherence and good insulating value is obtained.

Example 4 The coating is prepared as in Example 2, except that theprimer coat conforms with coating composition C of said U.S. Patent2,562,118. Good adherence and insulating values are obtained.

The resin coatings are poor thermal conductors. It is thereforedesirable to apply them as relatively thin coatings to prevent buildupof excessive internal temperature for specific heating element Wattchanges. The electric potential on the heating element surface to beinsu lated against by these coatings is only of the order of 1 volt andsince polytetrafiuoroethylene is practically an ideal insulatingmaterial adequate electrical insulation is provided by extremely thinpolytetrafiuoroethylene coatings. The important criterion is that thecoating thickness be such as to assure adequate adhesion and integrityand be within the capabilities of practical commercial application.

In view of our invention and disclosure variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the. art to obtain all or part of the benefits of ourinvention without copying the composition and method shown, and wetherefore claim all such insofar as they fall within the 4 reasonablespirit and scope of our claims.

Having thus described our invention what we claim as new and desire tosecure by Letters Patent is:

1. An immersion electric water heating element for cyclic operation inan electric water heater having a corrosion protective anode, forreducing deterioration of the anode and adapted to extend into the tankof the water heater, comprising a looped electric resistor, a resistorelectric insulating layer surrounding said electric resistor, a metallicsheath surrounding said resistor insulating layer, said sheath having anexterior surface of nickel, a sheath electric insulating layer ofpolytetrafiuoroethylene covering said nickel surface and adheringthereto, electric connections to the two ends of the electric resistor,and an insulating support for the electric connections.

2. A water heating element of claim 1, in which said sheath is ofcopper, having an exterior layer of nickel plated thereon.

3. An immersion electric heating element of claim 1, in which theexterior surface of said nickel surface has a matte finish.

4. An immersion electric heating element of claim 1, in which theexterior surface of said nickel surface has a coating of nickel oxide.

5. An immersion electric heating element of claim 2, in which theexternal surface of said nickel coating has a matte finish.

6. An immersion electric heating element of claim 2, in which theexternal surface of said nickel coating has a coating of nickel oxide.

References Cited UNITED STATES PATENTS 2,181,484 11/1939 Harris 338-2362,649,532 8/1953 Woodman 219-316 2,700,212 1/1955 Flynn et al 11771 X2,814,710 11/1957 Schuetze 219-245 X 2,816,207 12/1957 Boggs 219-523 X3,055,402 10/1961 Starger et al 156-583 X 3,103,446 9/1963 FitzSimmons11775 3,136,680 6/1964 Hochberg 16l189 RICHARD M. WOOD, PrimaryExaminer.

ANTHONY BARTIS, Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

1. AN IMMERSION ELECTRIC WATER HEATING ELEMENT FOR CYCLIC OPERATION INAN ELECTRIC WATER HEATER HAVING A CORROSION PROTECTIVE ANODE, FORREDUCING DETERIORATION OF THE ANODE AND ADAPTED TO EXTEND INTO THE TANKOF THE WATER HEATER, COMPRISING A LOOPED ELECTRIC RESISTOR, A RESISTORELECTRIC INSULATING LAYER SURROUNDING SAID ELECTRIC RESISTOR, A METALLICSHEATH SURROUNDING SAID RESISTOR INSULATING LAYER, SAID SHEATH HAVING ANEXTERIOR SURFACE OF NICKEL, A SHEATH ELECTRIC INSULATING LAYER OFPOLYTETRAFLUOROETHYLENE COVERING SAID NICKEL SURFACE AND ADHERINGTHERETO, ELECTRIC CONNECTIONS TO THE TWO ENDS OF THE ELECTRIC RESISTOR,AND AN INSULATING SUPPORT FOR THE ELECTRIC CONNECTIONS.